HOMOLOGIES

Leonard R. BrandBiology Department
Loma Linda University

There is a wide variety of animals, representing many different
types of structures, from one-celled protozoa to the most complex animal  man. There
are animals with skeletons inside their bodies and some with skeletons on the outside.
Some lay eggs and some bear live young; some are cold-blooded and some are warm-blooded.
Within each group there are many species, each a little different from the others. For
example, there are about 1200 species of rats and mice in the entire world. Some of those
species are so similar that it is difficult to tell them apart, but each one is different
in some way, and each species does not normally interbreed with any of the other species.
Zoologists arrange these animals in a standard classification scheme, beginning with the
simplest one-celled organisms and ending with man. In this classification system each type
of organism is placed next to those that are most similar to it.
The study of homologies plays an important part in determining which
organisms should be classified close together. When two animals have body parts that are
alike in their basic anatomy and develop along the same growth pathways when the animals
are embryos, these similar body parts are referred to as homologous parts. Consider the
arrangement of the bones in the forelimb of a man, a seal, a bat and a dog. Humans have
hands that are very agile for manipulating objects; seals' flippers are useful only for
swimming; bats have wings for flying, and dogs' feet are built for fast running. They all
look very different, yet they have the same basic arrangement of bones. Only the
proportions of the individual bones and the type of fleshy covering are different. A bat's
wing bears little resemblance to a human hand, and yet the wing membrane is supported by a
skeleton that is equivalent to our second, third, fourth and fifth fingers, but with very
elongated finger bones. The creation theory proposes that the vertebrate limb was designed
to be an efficient and adaptable structure, and then the same basic design was used for
all of the vertebrates. Only minor modifications, mainly in proportions and in the type of
fleshy covering, were needed to adapt this skeleton to the needs of each animal. These
different types of limbs are homologous, indicating that seals, bats, dogs and men should
be classified in the same group of animals.
However, according to the evolution theory, the fact that these animals
all have homologous limbs is considered to be evidence that they have all evolved from
common ancestors. It is often believed that they would not have homologous limbs unless
they had inherited them from common ancestors. However, these homologies are also what we
would expect to see if the vertebrates had all been designed and created according to a
common vertebrate body plan. Consequently, homologies in anatomy are not really evidence
for or against evolution.
There are also homologies in physiology, biochemistry and embryology.
The principles are the same in these fields as for homologies in anatomy. Similarities in
physiology or similar developmental pathways in embryos are often considered to be
evidence of evolution from a common ancestor, but they are also what we would expect to
see if all life was created by a single intelligent Designer. Also in the biochemical
structure and functions of cells, there are many features that are virtually the same
through the animal and plant kingdoms. For example, all living things, both plants and
animals (excluding some viruses), have chromosomes containing DNA. This DNA contains the
genetic code that determines the entire structure and physiology of the organism. The
basic details of this mechanism are the same in all living things. This is considered by
some to be evidence that all living things evolved from a common ancestor, but we can also
consider it to be evidence that all living things were designed by one intelligent
Designer who used the same exquisitely designed genetic mechanism for all.
The details of the evolution theory of the history of life are based
largely on these homologies between organisms. All plants and animals are arranged in the
classification system with the simplest ones first, and then more and more complex
organisms. Those who accept the evolution theory believe that this arrangement is the
order in which the animals evolved, from simple to complex. Organisms with the most
similarities, or homologies, are placed closest together in the classification system.
From this classification scheme, phylogenetic trees are constructed. Phylogenetic trees
are diagrams representing the presumed evolutionary pathways along which organisms have
evolved.
If we would compare many different types of wheeled vehicles, we would
find that they also have many homologous parts and that they can be arranged in a sequence
based on these homologies. For example, they all use the principle of the wheel. Most of
them also use levers in some way, and several use energy produced by the internal
combustion of fuel. Using this information, we can construct a "phylogenetic
tree" by following the same principles used in making an animal or plant evolutionary
tree. Of course no one would say that this means that cars evolved from two-wheeled carts.
The different vehicles have homologous parts because they were all designed to operate
under the same natural laws. Certain design concepts are used in several different
vehicles and adapted to meet the different functional requirements of each one. They can
be arranged in a sequence of simple to complex because they all are designed to serve
different functions, and thus their structural requirements are quite different. The
result is a wide diversity of types, differing in structural complexity, and each well
suited to perform its unique function.
When we apply these same principles to living things, we can develop an
interpretation that is consistent with both the biological data and the concept of
creation. Hence one of the most commonly used arguments for evolution utilizes data that
are not especially supportive of the theory, but fit equally well the concept of creation.